This invention relates to a method for drilling offshore wells and, more particularly, a method for drilling in deep water. The need to develop new reserves of oil and gas continues to drive hydrocarbon recovery operations into progressively deeper water and drives the need to reach ever deeper reservoirs. One problem encountered in deep water development is the cost of providing suitable platforms and other offshore facilities and the economics generally suggest getting the maxium advantage from each facility placed. The traditional approach includes extensive directional drilling to drain the largest area of reservoir possible. These influences can dictate that wells be drilled to a total length of four miles and more.
Whether drilling onshore or offshore, it is necessary to seal off the borehole wall as drilling progresses in order to control the geopressures, maintain the borehole wall and isolate the zones. This is temporarily accomplished for each interval of drilling with the filter cake deposited by drilling mud which also serves to cool the bit and flush away cuttings. After a given interval has been drilled and the drill string pulled, the borehole is permanently sealed with casing which is cemented to the borehole wall. Thereafter, the next interval is drilled through the last casing and, subsequently, sealed off with another casing which is concentrically run through the last previous casing, hung off and cemented in place. Thus, each interval results in another concentric casing, each progressively reducing the size of the wellbore available for further drilling, completion and production operations over the life of the well.
Directional wells drilled for extended reach make these problems worse because the reduced interior diameter is particularly difficult to run casing through when there are bends and/or shallow angle sections in the borehole.
Further, the offshore environment makes these operations more difficult because drilling in deep water from surface facilities requires an artificial "borehole" to be maintained from the sea floor to the surface facilities. This is provided by a hollow tubular member called a riser. The riser returns cuttings to the surface and holds a vertical column of drilling mud which maintains the hydrostatic head necessary to provide pressure controlled access to the wellbore while pulling the drill string, setting casing or running other tools.
However, the diameter of the riser itself is another limiting factor in offshore drilling operations. This limitation has, in past practice, led to removing the riser for setting at least the first conductor casing in order to set an early casing which is too large to pass through the riser. Alternatively, the conductor interval has been drilled without a riser. However, in either event the conductor casing included an integral high pressure housing and the surface interval was constrained by both the housing and a blowout preventer ("BOP") on the riser.